The present invention generally relates to the field of solid state relays. More specifically, the present invention relates to improving the switching characteristics of solid state relays whose output section is built on MOSFETs. Improving these characteristics will allow us to decrease dynamic loses on solid state relays so an output stage having a number of MOSFETs in parallel can be built.
Solid Solid State Relays (AKA SSRs) with the output stage built on power MOSFETs are well known devices on today's electronics market because they are easily controllable and can be used in applications to conduct AC or DC currents, or both. The MOSFETs switches manufactured today are designed to have very little drain-source resistance in the conductive “ON” state, allowing the conduction of very high currents with minimal power losses on the device.
There are two main types of power losses on MOSFETs static and dynamic.
Static losses are the result of the electrical resistance of the MOSFET's drain-source channel in the conductive state and power losses on the SSR's output stage built on MOSFETs will be determined as the function of this resistance and operational current flowing through the drain-source. These losses will limit the SSR's maximum operational current and its maximum operational temperature without using any heat sinks. For example, the IRFB4410 MOSFET manufactured by International Rectifier™ has 10 milliohm drain-source resistance in the conductive state. Power losses on this particular device will be equal to 1 watt when 10 A of current flows through the drain-source channel. According to the International Rectifier™ technical specifications, dissipating 1 watt on the IRFB4410 MOSFET having TO-220 package without using a heat sink will increase the MOSFET's junction temperature up to 80-85° C. in a condition when the ambient temperature is 25° C. It will be increased up to 140-145° C. in a condition when the ambient temperature is 85° C., which is the standard maximum industrial ambient temperature level.
Dynamic losses on the MOSFET will occur during the time when the device is switched to the “ON” from the “OFF” state and vise versa. Thus the slower switching time will cause larger dynamic losses as well as bigger losses of power dissipation on the MOSFET. Taking in consideration the MOSFET's gate-source capacitance value there are certain difficulties in achieving the fast charging and discharging time of this built into the MOSFET. Fast switching time is especially difficult to achieve when the input control signal source is used to switch the MOSFET into a conductive or nonconductive state has the limited output power.
Static losses on the SSR's power output stage built on the MOSFET could be decreased by connecting the same type of MOSFETs in parallel. For example, to decrease the static losses by two times it would require the doubling of the same type of MOSFETs connected in parallel, thus, the MOSFET′ gate-source capacitance will be increased proportionally and will be equal to the sum of the gate-source capacitance of each MOSFET connected in parallel. Knowing that the gate-source capacitance of the power MOSFET is approximately equal to 3-10 nF, the resulting common gate-source capacitance of the two or more MOSFETs connected in parallel could reach tens of nanoFarads. As a result of this, dynamic losses on the device will be increased correspondingly.
There are number of ways to control the SSR's output stage built on power MOSFETs, but what all of them have in common is that generated voltage needs to be applied to the MOSFET's gate-source to reliably switch it. The only difference is what kind of device will be used to generate this voltage and how the primary control circuitry is isolated from the secondary one, which in turn will control the SSR's power MOSFET's output stage.
This control voltage can be applied to the MOSFET′ GATE-SOURCE by using a photovoltaic device or DC/DC converter built using a transformer.
Because of their lack on generated output energy, a photovoltaic device can only be used to control the low current MOSFETs having low GATE-SOURCE capacitance value. In this case the output switching time will be in range from ones to hundreds of milliseconds.
Using a DC\DC converter built on an isolation transformer to generate voltage to control the MOSFET is more preferable because these converters are able to deliver much higher energy to charge the MOSFET's GATE-SOURCE capacitance than a photovoltaic device. This is the reason why this method of MOSFET control will allow the building of more powerful SSRs.
One of the examples of using DC\DC converters can be found in U.S. Pat. No. 4,438,356 by Kenneth Fisher, Assignee International Rectifier Corp. In this case the MOSFET control is achieved by the transformation of energy from the primary control circuitry to the secondary by using a DC/DC converter. This DC/DC converter builds up the energy directly on the GATE-SOURCE capacitance of the MOSFET with every cycle by charging it. It will take a certain amount of cycles to build up enough energy directly on the MOSFET's input GATE-SOURCE capacitance charging it to switch the MOSFET from the “OFF” to the “ON” state and in this case, the switching time span can reach tens of microseconds. The discharging circuitry is built on JFET and the JFET's channel resistance is usually tens of ohms in a best case scenario, thus the required time to switch the power MOSFET can reach hundreds of microseconds.
In a situation when the output stage is built by connecting a plurality of MOSFETs in parallel, this time will be increased significantly, therefore escalating the dynamic losses and decreasing the reliability of the SSR itself.
The purpose of this patent is to improve the switching characteristics of the SSR's MOSFET output control circuitry. Improving these characteristics will allow the decreasing of the dynamic losses, thus allowing the connecting of a number of MOSFETs in parallel to build more powerful and reliable SSRs
Our goal is to achieve the fastest possible MOSFET's switching time to the ON and the OFF state. This can be achieved only by very fast charging and discharging of MOSFET's gate-source capacitance.
The main idea as how to significantly improve this charging and discharging time is to charge some additional external capacitors with a much bigger capacitance value than the MOSFET's gate-source own capacitance to the voltage level high enough to control the MOSFET′ state. Energy received from the isolation transformer is used to charge this additional capacitor circuitry.
After the voltage level on the external capacitors reach a level high enough to change the MOSFET's state, it will be applied by fast electronic switches to the MOSFET's gate-source. Voltage from these capacitors will charge the MOSFET's input capacitance thus changing its state from OFF to ON. In other words, we can treat these external capacitors as some sort of accumulator from which the energy is transferred by fast electronic switches to the MOSFET's gate-source and thus switching the MOSFET to the ON state.